Activation of the coagulation system, culminating in thrombus formation, is the primary defense mechanism of the host against blood loss and occurs in association with a number of diseases. Fibrinolysis is the necessary compensatory pathway required for dissolution of the fibrin clot, and effective fibrinolysis is required at both extravascular and intravascular sites of fibrin deposition. In addition, fibrinolysis is an important mediator system with the enzymes and products generated influencing coagulation, other biological pathways and the functions of a variety of cells. In normal hemostasis, these influences are regulated, but in the wide variety of disease associated with excessive coagulation and fibrinolysis, they can contribute directly to pathogenesis. Two primary pathways for fibrinolysis have been identified: 1) the classical system in which activators of both humoral and cellular origin convert plasminogen to plasmin; and 2) the alternative pathway associated with leukocytes in which elastase and cathepsin G are the primary fibrinolytic enzymes. In each pathway, the activities of the enzymes are regulated by specific inhibitors including alpha 2 antiplasmin and alpha 1 antitrypsin. Efficient and well-regulated fibrinolysis is achieved in vivo by the balance of the activity and inhibitory capacities of each pathway and by participation of both pathways in the fibrinolytic process. Pathological fibrinolysis then occurs as a result of imbalances within or between the fibrinolytic pathways. In this application, we propose to investigate the mechanisms regulating fibrinolysis in normal hemostasis and the events leading to and associated with pathological fibrinolysis. Selected molecules and aspects of the two pathways will be subjected to detailed biochemical and immunochemical investigations. Specific assays will be developed which permit detection, quantitation, and discrimination of the active fibrinolytic enzymes, their free inhibitors, and the enzyme:inhibitor complexes. Additionally, systems will be developed to characterize fibrinolytic products and to permit identification of the pathway leading to their generation. These approches will be applied to the analysis of selected diseases to assess the contribution of the pathways to fibrinolysis.